1. Field of the Technology
The present invention relates to data transmission technologies in an Optical Transport Network (OTN), and particularly, to a method and a device for transmitting low rate signals over the OTN.
2. Background of the Technology
Aiming at the rapid development of OTN, the International Telecommunication Union—Telecommunication Standardization Sector (ITU-T) has issued OTN series recommendations ITU-T G.709, G.798 and G.87X, and OTN products are coming into commercial use. Among the recommendations, G.709, put forward in February 2001, is of particular significance and has laid the technical foundation of optical internetworking. The core of G.709 recommendation is a digital wrapper technology, in which a special frame format is provided to encapsulate client signals into the payload unit of a frame and provide overhead (OH) used for Operation, Administration, Maintenance and Provision (OAM&P) in the head of the frame and Forward Error Correction (FEC) bytes at the end of the frame.
The standard frame format adopted by digital wrapper is as shown in FIG. 1. It can be seen that the standard frame in a digital wrapper technology adopts a frame format of 4 rows×4080 columns. The 16 columns in the head of a frame are OH bytes, the 256 columns at the end of frame are FEC check bytes, and the 3808 columns in the middle are payload. In the OH bytes in the head of the frame, the first 7 columns in the 1st row are Frame Alignment Signal (FAS); the 8th to 14th columns are level k Optical channel Transport Unit (OTUk) OH bytes, and different values of k correspond to different transmission modes of different rates; the first 14 columns in the 2nd to 4th rows are level k Optical channel Data Unit (ODUk) OH bytes, the 15th column and 16th columns are Optical channel Payload Unit (OPUk) OH bytes, in which k=1, or 2, or 3. The 7th byte in the FAS is a Multi-Frame Alignment Signal (MFAS) used for indicating the OH allocation when multiple client signals are carried in a TDM mode.
OTUk OH bytes provide monitoring functions for the status of signals transmitted between the 3R (Reamplification, Reshaping, and Retiming) regeneration points in the OTN, including three portions: the overhead bytes for Section Monitoring (SM), the overhead bytes for inter-terminal General Communication Channel 0 (GCC0), and Reserved (RES) bytes.
ODUk OH provides tandem connection monitoring, end-to-end path supervision and adaptation of client signals, as well as plenty of overhead bytes (columns 1 to 14 of rows 2-4) to achieve the above functions, including bytes for Path Monitoring (PM) OH, Tandem Connection Monitoring (TCM) OH, GCC 1 (General Communication Channel 1) OH and GCC 2 OH, Automatic Protection Switching/Protection Communication Channel (APS/PCC) OH, Fault Type and Fault Location (FTFL) message, and Experimental OH (EXP).
An OPUk consists of payload into which client signals are mapped, and related OH including a Payload Structure Identifier (PSI), justification bytes and Mapping Specific Overhead; the value of the PSI ranges from 0 to 255 according to the indication of an MFAS, and the PSI[0] indicates the Payload Type (PT) of a client signal and the rest bytes are reserved bytes (RES) for future extension.
An OTN is a major technology for the lower layer transmission in a future network, and the key to the optimized application of the OTN includes signals of various rate levels over the OTN and corresponding carrying and mapping technologies. High rate signals may directly be mapped into signals of corresponding rate levels for transmission by adopting an ODJk/OPUk/OTUk provided by the digital wrapper technology, and the signal uplink/downlink and signal management can thus be performed. With respect to low rate signals, since direct rate adaptation is impossible, further plans shall be sought to map and multiplex the low rate signals into the ODUk/OPUk/OTUk of various rate levels as well as to solve the problems in transmission efficiency, transmission performance, equipment complexity, operation cost, etc.
At present, there are the three following methods for mapping client signals into the OTN.
(1) Mapping signals of CBR (Constant Bit Rate) 2G5, CBR10G, and CBR40G into an OPUk: constant bit rate signals of CBR2G5—2488320 kbit/s±20 ppm, e.g. STM-16; constant bit rate signals of CBR10G—9953280 kbit/s±20 ppm, e.g. STM-64; constant bit rate signals of CBR40G—39813120 kbit/s±20 ppm, e.g. STM-256. The mapping may be performed according to two different modes (asynchronous and bit synchronous). In the asynchronous mode, a local clock is not associated with client signals and a positive/negative/zero justification scheme is used. In the bit synchronous mode, the clock derived from client signals is used.
(2) Mapping Asynchronous Transfer Mode (ATM) signals into an OPUk: a constant bit rate ATM cell stream with a capacity that is identical to the OPUk payload area is created by multiplexing ATM cells so as to be mapped into the OPUk, wherein the rate is adapted by either inserting idle cells or discarding cells during the multiplexing. The ATM cell information field should be scrambled before mapping.
(3) Mapping of GFP (General Framing Procedure) frames into an OPUk: mapping GFP frames into a continuous bit stream matching the OPUk by inserting idle frames at the GFP encapsulation stage, and scrambling is also performed during the encapsulation. Other signals may be mapped into the OPUk as well, such as client signals, test signals, and common client bit stream signals.
According to the OTN recommendation at the present, a data signals solution is achieved by adapting data units to an OPTED by means of a GFP, which is suitable to high rate signals; but with respect to low rate signals, e.g., CBR155/CBR622 signals in Gigabyte Ethernet (GE) signals, Fiber Connection (FC) signals and Metropolitan Area Network (MAN), the solution brings many problems to the transmission of low rate signals over the OTN, including low bandwidth utilization, low transmission efficiency, poor performance in transparent uplink and downlink transmission and end-to-end management, difficulties in line maintenance, complicated devices, large volume of calculation, high cost, etc.
Since the smallest dispatching granularity of an OTN is level 2.5G (for different k, the rate is: 2.5G when k=1, 10G when k=2, and 40G when k=3); in the prior art low rate signals such as GE signals are adapted to an OPU1 and then dispatched by an ODU1, which results in unmatched rate and bandwidth resource waste. When two GE signals are adapted to Synchronous Digital Hierarchy (SDH) VC-7V virtual concatenation via a GFP, multiplexed into STM-16 signal, and mapped into an OPUk/OTN, the dispatch function of an ODUk on an OTN can not act on GE signals and an extra virtual concatenation process on the SDN layer will be added; when two GE signals are adapted to a part of an ODU1, the performance monitor function of the ODU1 cannot recognize the error performance of a single GE signal. Such practice makes it impossible to implement transparent uplink and downlink transmission and end-to-end management for individual low rate signals and the system complexity will also be greatly increased.
A variety of low rate signals, e.g., GE and FC signals as a type of client signals, will continue to exist in backbone networks and MANs for a long time, especially in backbone networks. An OTN transmission technology is one of the key technologies for lower layer transmission in the future, and therefore it is urgent to find a way to enable the transparent transmission of GE level low rate signals over the OTN, end-to-end management of low rate signals and flexible signals uplink and downlink at intermediate nodes.